Taking an internal measurement, say for example, measuring the exact width of a room between two walls is problematic. First it is necessary to maintain the outside portion of the hook on the tape against one of the two walls while the tape is extended. To prevent the tape from bending under the force of gravity it is necessary that the tape and the tape measure be maintained in an upright position. Then it is necessary, while holding the back of the housing of the tape measure against the other of the walls to read the tape measure. Typically a tape measure has a lateral slot which is about ¼ inch high through which the tape is pulled from the housing. One must read the measurement lines and numerals on the tape where it begins to project from the housing. Because the housing is about ¼ inch above the tape one should align their eye on a line projected along a front side wall of the housing which is perpendicular to the tape to avoid parallax and thereby obtain an accurate reading. The problem with doing this is that it is impossible to position one's eye perpendicular to the tape above where the tape projects from the housing when the tape measure is adjacent to the wall. One solution is to extend the tape around a corner on the adjacent wall, and then read the tape at a midpoint in the arc where the tape bends around the corner. But to do this with a reasonable degree of accurately one must ensure that they read the tape when there eye is on a line which projects perpendicularly from a midpoint of the tape bending around the corner. This is time consuming and problematic.
Some tape measures are numerated to allow one to read both outside and inside measurements. The set of numerals for inside measures includes the extended length of the tape plus and additional few inches for the length of the housing on the tape measure when the back thereof is held against the other wall. This simplifies having to add this additional length onto the measured projection of the tape from the housing. But the problem with this is that when measuring a longer length, one may inadvertently read from the wrong set of numerals. One solution to this is to place one set of numerals on the convex side of the tape. But these are subject to excessive wear when the tape moves in and out of the housing. Another problem is that it is just as difficult to read the underside of the tape accurately, especially when the tape measure is positioned adjacent to the other wall. What is needed is a way to so clearly distinguish between the sets of numerals so that they could not inadvertently be mistake even at the end of a long day when a user is not alert.
Yet another problem is that tape measures are too easily brushed off a user when clipped on a belt. If a tape measure is brushed off and falls on a hard surface, or from a ladder, it is likely to be damaged.